Voltage Accuracy of Switching Power Supplies

The switching power supply features a feedback pin that can be utilised to set the output voltage. This output voltage is regulated by the power supply circuitry. However, the question arises: how precise is this voltage regulation?

Figure 1 illustrates a buck-type switching power supply.

It employs an existing, typically unstable power supply voltage (input voltage) to generate a regulated output voltage that is as accurate as possible.

Figure 1. Circuit Structure of a Buck Switching Power Supply

The accuracy of the output voltage is determined by multiple influencing factors and tolerances.

DC tolerance

DC regulation deviation encompasses the accuracy deviation of the reference voltage incorporated within the voltage converter (Figure 1). The reference voltage is established by the integrated circuit designer, exhibiting only minimal deviation across the power supply voltage range, IC manufacturing tolerances, and operating temperature range. DC tolerances are specified in the voltage regulator IC's data sheet. These tolerances may be defined in several ways, expressed as the reference voltage's linearity, the output voltage's load regulation, or the stabilised feedback voltage. Beyond the pure accuracy of the reference voltage, the latter also encompasses other IC-specific DC tolerances at the feedback pin.

DC tolerances also encompass deviations in the resistor values within the resistor divider connecting the stabilised output voltage to the feedback pin (Rfb1 and Rfb2 in Figure 1). Such resistors are available with multiple tolerance specifications. Typically, resistor tolerances approach the specified maximum tolerance. During production, high-quality resistors with tighter tolerances are selected and marketed as high-precision components.

Output voltage ripple

In switching power supplies, a certain degree of voltage ripple invariably persists in the regulated output voltage. This voltage ripple is determined by fluctuations in the inductor current of the buck regulator, alongside the output capacitance value and its equivalent series resistance (ESR). Employing coils with high inductance values and large output capacitors with low ESR can reduce voltage ripple; however, switching power supplies cannot entirely eliminate output voltage ripple.

Load transient

The third factor affecting the accuracy of the generated voltage is its behaviour during load transients. Should the load current change, the generated voltage may deviate upwards or downwards. The greater the magnitude of the load transient and the faster it occurs, the larger the short-term deviation of the output voltage from the setpoint.

To minimise the impact of this factor on the error of the generated voltage, the control loop must be sufficiently fast. ADI's LTpowerCAD® tool optimises the control loop of power converter circuits.

Figure 2 illustrates the load transient for the LT8642S. The load current increases from 100 mA to 5 A within 500 ns and then rapidly decreases after 250 μs. The voltage peak is approximately 27.5 mV, and the voltage trough is approximately 26.4 mV.

Figure 2. Measuring load transients in the time domain using LTpowerAnalyzer

Load transients were measured using ADI's LTpowerAnalyser. This hardware facilitates straightforward measurement of load transients at varying current levels and step times.

Figure 3 illustrates the various factors influencing switching power supply tolerance.

Figure 3. Factors Affecting the Accuracy of Power Supply Output Voltage

Conclusion

When selecting a power supply circuit, the first step is to determine the voltage accuracy required for the specific application. This is crucial as it directly influences component selection. For instance: if DC accuracy is a priority, focus on selecting the power converter IC and resistor voltage divider; if output voltage ripple and load transients are of concern, choose suitable inductors and output capacitors. Furthermore, the circuit's control loop should be optimised to handle load transients as effectively as possible, for example by employing the lowest possible voltage offset.